Question: Why do we add the paragraph about Shabbos when we read about Rosh Chodesh on Rosh Chodesh during krias haTorah but the Mishna Berura says there isn’t a maftir aliyah about Shabbos as we have for all other chagim because the paragraph for Shabbos is too short and you can’t add pesukim of a different topic?
A. The Mishna Berura (283:1:1) says the reason why we don’t take out a second Torah every shabbos to read the mussaf parsha for shabbos in parshas Pinchas is because it only has 2 verses. An aliyah is read with no less than 3 verses, and to combine verses which is not the topic of the day one cannot do.
Answer: The Dirshu (1) quotes the Taz (Orach Chaim 423:1) who quotes a Ra”n in Megilla 12b in the Rif that says the verses of Shabbos are part of the topic if Rosh Chodesh because Rosh Chodesh sometimes falls out on Shabbos.